1. Field of the Invention
This invention relates to a field emission cathode known as a cold cathode.
2. Description of the Related Art
When the electric field at a surface of a metal or semiconductor is as large as 10.sup.9 V/m, electrons pass through the potential barrier because of the tunnel effect, thus entering an evacuated space at room temperatures. This phenomenon is called field emission. The cathode for emitting electrons under such a principle is referred to as a field emission cathode (FEC) or a field emission element.
Recently, a plane-type field emission cathode formed of micron-size field emission elements could have been fabricated by fully using the semiconductor microprocessing techniques. The structure, in which plural field emission cathodes are arranged on a cathode substrate, can be used as electron supplying means for flat display devices or various electronic devices because emitters therein can radiate electrons onto the fluorescent surface.
FIG. 14 is a perspective view showing a field emission cathode called a Spindt type cathode (hereinafter sometimes referred to as FEC) as an example of the above-mentioned field emission cathode. Referring to FIG. 14, a cathode electrode layer 101 is formed on a cathode substrate 100. A resistance layer 102, an insulating layer 103, and a gate electrode 104 are sequentially formed over the cathode electrode layer 101. Then, cone emitters 115 are respectively formed in openings formed in the gate electrode layer 104 and the insulating layer 104. The tip of each cone emitter 115 is viewed from the opening in the gate electrode layer 103.
In the FEC, the cone emitter 115 and the gate electrode layer 104 can be spaced on the order of submicrons by using a microprocessing technique for fabricating integrated circuit devices. Consequently, the emitter can emit electrons when a low voltage of several ten volts is applied between the emitter 115 and the electrode 104.
As shown in FIG. 14, the anode substrate 116, on which a fluorescent substance layer is coated, is disposed above the cathode substrate 100 on which plural field emission cathodes are formed in a matrix pattern. In the display device, electrons can be emitted by applying the voltage V.sub.GE and the voltage V.sub.A, thus glowing the fluorescent substance layer.
The reason that the resistance layer 102 is disposed between the emitter 115 and the cathode electrode layer 102 is as follows:
In the Spindt-type field emission cathode, since the spacing between the emitter 115 and the gate electrode layer 104 is set to a very small value, dust often makes a short circuit between an emitter 115 and a gate electrode layer 104 in a fabrication process. When one short circuit is formed between a gate electrode layer 104 and an emitter 115, the voltage can not be applied between all the remaining gate electrode layers 104 and the remaining emitters 115, so that the whole system becomes inoperable.
When the field emission cathode is initially operated, gases are locally released, thus often causing a discharge between the emitter 115 and the gate electrode layer 104 or anode electrode 116. As a result, the cathode electrode layer 101 may be sometimes destroyed due to a large current flowing therein.
Moreover, since a specific emitter which tends to emit electrons easily of the many emitters 115 emits intensively electrons, it accepts an excessive current locally. This often causes an abnormal bright spot on the screen.
The resistance layer 102 is disposed between the emitter 115 and the cathode electrode layer 101. An excessive electron emission of a specific emitter 115 causes increasing the current flowing through the emitter 115. The resistance layer 102 drops the voltage so as to suppress electron emission of the cone emitter 115, so that the emitter 115 can be prevented from emitting electrons uncontrollably. The resistance layer 102 can prevent the current from concentrating to a specific emitter 115, thus leading to the improved yield of FEDs in the manufacture and the stable operation of FEDs.
However, even when the field emission cathode has a resistance layer disposed between the cathode electrode and the emitter, as shown in FIG. 14, it is difficult to equalize the number of electrons emitted from a great number of emitters. A short circuit between a gate electrode and an emitter often causes a defect in the corresponding line.
In an emitter array where plural emitters form the electron source corresponding to each pixel, a short circuit between an emitter and a gate electrode makes it difficult to supply a sufficient current. Hence, there is the disadvantage of decreasing the brightness at only the specific defect spot in a display.